April 29, 2017 at 2:44 am

The discovery of large amounts of genetic

variation in nearly all populations led to the

formulation of a different question: How is

genetic variation maintained? In many cases,

after all, natural selection removes genetic

variation by eliminating genotypes that are less

fit.

Many factors act to increase or maintain the

amount of genetic variation in a population.

One of these is mutation, which is in fact the

ultimate source of all variation. However,

mutations do not occur very frequently, only at

a rate of approximately one mutation per

100,000 to 1,000,000 genetic loci per generation.

This rate is too slow to account for most of the

polymorphisms seen in natural populations.

However, mutation probably does explain

some of the very rare phenotypes seen

occasionally, such as albinism in humans and

other mammals.

A second factor contributing to genetic

variation in natural populations is selective

neutrality. Selective neutrality describes

situations in which alternate alleles for a gene

differ little in fitness. Because small fitness

differences result in only weak natural

selection, selection may be overpowered by the

random force of genetic drift. Alleles whose

frequencies are governed by genetic drift

rather than by natural selection are said to be

selectively neutral. Under neutrality, allele

frequencies vary over time, increasing or

decreasing randomly. Over long periods of

time, random fluctuations in the relative

frequencies of different alleles may result in

some being eliminated from the population.

However, genetic polymorphisms are long-

lived, and novel neutral alleles may arise

continually through mutation.

Finally, several forms of natural selection act to

maintain genetic variation rather than to

eliminate it. These include balancing selection,

frequency-dependent selection, and changing

patterns of natural selection over time and

space.

Balancing selection occurs when there is

heterozygote advantage at a locus, a situation

in which the heterozygous genotype (one

including two different alleles) has greater

fitness than either of the two homozygous

geno-types (one including two of the same

allele). Under heterozygote advantage, both

alleles involved will be maintained in a

population.

A classic example of heterozygote advantage

concerns the allele for sickle-cell anemia.

Individuals who are homozygous for the sickle-

cell allele have sickle-cell anemia, which causes

the red blood cells to become sickle-shaped

when they release oxygen. These sickle-shaped

cells become caught in narrow blood vessels,

blocking blood flow. Prior to the development

of modern treatments, the disease was

associated with very low fitness, since

individuals usually died before reproductive

age.

Heterozygotes, however, have normal, donut-

shaped blood cells and do not suffer from

sickle-cell anemia. In addition, they enjoy a

benefit of the sickle-cell allele, which offers

protection from malaria. Consequently,

heterozygous individuals have greater fitness

than individuals who have two copies of the

normal allele. Heterozygote advantage in this

system is believed to have played a critical role

in allowing a disease as harmful as sickle-cell

anemia to persist in human populations.

Evidence for this comes from an examination

of the distribution of the sickle-cell allele,

which is only found in places where malaria is

a danger.

Another form of natural selection that

maintains genetic variation in populations is

frequency-dependent selection. Under

frequency-dependent selection, the fitness of a

genotype depends on its relative frequency

within the population, with less-common

genotypes being more fit than genotypes that

occur at high frequency.

Frequency-dependent selection is believed to

be fairly common in natural populations. For

example, in situations where there is

competition for resources, individuals with

rare preferences may enjoy greater fitness than

those who have more common preferences.

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